(1) Field of the Invention
This invention relates generally to the field of water treatment, and more particularly to systems used to separate and remove both floating and dissolved volatile organic compound (VOC) contaminants from groundwater sources.
(2) Description of the Related Art
Environmental regulations require the cleanup of spills from underground storage tanks containing fuels or other light density hazardous materials at commercial or industrial locations which impact groundwater aquifers. Such rules mandate removal of immiscibles floating on the aquifer surface and the solutes that have dissolved into the water.
Known methods for removing floating VOCs include the use of bailers, skimmer pumps and other devices. Proposed methods for removing dissolved VOC contaminants from water include such techniques as: pumping and air stripping, vaporization of the entire liquid stream with selective recondensation of the water, hydrocarbon contaminant adsorption, biological reduction by aggressive organisms, and chemical treatments.
A common problem in the cleanup of volatile hydrocarbon materials which have leaked from underground storage tanks is containment and recovery of spilled contaminants. After the contaminants have migrated vertically downward through the vadose zone, a portion of said contaminants will float on the water table as a separate phase and a portion will dissolve into the groundwater.
The areal spread of contaminants about a groundwater aquifer is known as a contaminant plume and is defined by hydrogeological studies. Once a plume is defined, attempts are made to control or confine further spread of the plume. However, plume confinement is often complicated by the fact that aquifers act as underground rivers and may exhibit substantial horizontal movement. In addition, aquifer movement is affected by nearby water wells which may draw the plume toward those wells. Drinking water supplies can easily become polluted by these migratory contaminants. Therefore, swift recovery of the plume is a frequent tactic in groundwater cleanups to minimize migration of the spilled contaminants.
Recovery of the liquid phase, or "free product", that floats freely on the surface of the water table is particularly troublesome in the cleanup of groundwater contaminated by light density VOCs. Extraction of free product from groundwater is a complex procedure complicated by the logistics of accurately setting removal equipment at the point of density change where the free product and water interface occurs. This positioning is required so that the floating free product above this point can be recovered.
It is difficult to maintain the extraction equipment at the appropriate level as the water table elevation changes and to extract the free product quickly before the contaminant plume moves away from the recovery well. Additional problems encountered in the recovery process are: voluntary segregation of the floating free product into multiple floating pools which are not interconnected, limited accessibility to the water table due to the small physical size of the recovery wells, and restrictions upon the number of extraction wells imposed by the high cost of drilling and installing the wells.
With the bailer systems previously mentioned, either a bucket type bailer or a pipe bailer with a one-way valve is repeatedly lowered to the water table to retrieve the free product. Both automated and manual bailing systems are used, however, bailing is slow and the automated bailers are prone to mechanical problems.
With skimmer pumps, either a fixed position pump or a floating pump is lowered into a well and positioned at the free product interface at the top of the water phase. The floating free product is then pumped or skimmed. Like bailing, skimming is also a slow recovery process and considerable difficulty exists in controlling the appropriate depth for fixed position skimming pumps due to changes in water table elevation. With each change in water table elevation, fixed position skimming pumps must either be manually repositioned or automatically repositioned using sophisticated and expensive telemetry controls. Additionally, both fixed position and floating skimmers are prone to foul due to sand, fine silt or other contamination from the aquifer.
A common method of plume control includes drilling one or more wells into an aquifer and pumping water therefrom with submersible pumps. The removal of water draws the migrating plume toward the recovery wells thereby localizing said plume. The contaminated groundwater is pumped to the surface grade level from within the well where said water is treated and decontaminated. During the process, water is pumped at a sufficient volume and rate to draw down the water level of the aquifer and create a vortexial cone of depression above the pump inlet. Freely floating VOCs migrate along the surface of the water table and are drawn into the cone of depression by gravity where said VOCs are maintained for removal by bailing or skimming.
While undesirable, the free product floating within the cone of depression may be occasionally drawn into the recovery pump in a bailing or skimming process if the pumping rate is too high or if the drawdown capacity of the aquifer changes. Cavitation may also occur with excessive pumping if the rate at which water is removed by the pump is greater than water can flow to the pump within the aquifer.
U.S. Pat. No. 4,892,664 to MILLER discloses a system for decontamination of sites where organic compound contaminants endanger the water supply. An apparatus is shown in MILLER that includes an air stripper and catalytic converter. Volatile organic compounds are first stripped from groundwater so that a VOC laden vapor is created that is then burned through a catalytic stage that oxidizes the organic compounds.
U.S. Pat. No. 4,722,800 to AYMONG discloses an oil-water separator. AYMONG shows a baffled holding tank that is used to separate both solids and immiscible liquids from a solution.
U.S. Pat. No. 4,526,692 to YOHE discloses a process for purification of contaminated groundwater. YOHE shows a system used to purify a domestic water supply. In this process, water is brought up from a well, purified, and then returned to the well beneath the ground.
U.S. Pat. No. 4,139,463 to MURPHY discloses a method of and means for oily water separation. MURPHY shows a two tank system used together with a suction pump to withdraw oil and water from a ship's bilge or sump. The two tanks are then used to allow the heavier water to separate from the lighter fluids.
Other patents which disclose means for separating petroleum products from water include:
U.S. Pat. No. 745,519 Pravicha, PA1 Russian Patent No. 1313483 Mutin, PA1 Russian Patent No. 1414402 Didenko.